1. Field of the Invention
The present invention relates to semiconductor color image sensors such as CCD (Charge Coupled Devices) and CMOS (Complementary Metal Oxide Semiconductor) image sensors. In particular the invention concerns improvements in or relating to the response of such sensors at shorter wavelengths.
2. Description of the Related Art
As is known semiconductor image sensors comprise an array of semiconductor photo-sensors which measure the intensity of light incident on them. In a color image sensor the contribution of red (R), green (G) and blue (B) light components are measured for each pixel of the image using a color filter array (CFA) which comprises an array of red, green and blue optical bandpass filters. Individual filter elements of the CFA overlie a respective photo-sensor such that each photo-sensor measures light intensity for a given color of light. In such image sensors a single image pixel thus comprises at least three photo-sensors for measuring red, green and blue light contributions. A partially exploded schematic representation of a color image sensor 10 is shown in FIG. 1 and comprises a photo-sensor array 12, a CFA 14 and an array of micro-lenses 16.
Many color image sensors utilize a Bayer Pattern mosaic CFA in which there are twice as many green filters as there are red and blue filters. This concept is illustrated in FIG. 2 in which the CFA 20 has in a first row 22 a pattern which alternates between red and green filters, and in a second row 24 a pattern which alternates between green and blue filters. The need for having twice as many green filters as either red or blue filters is because the human eye is more sensitive to green light than to the other two colors of light. Thus, the accuracy of green in a color image is more important. Since a color filter 14A, 14B, 14C in the CFA layer 14 (FIG. 1) covers a respective photo-sensor 12A, 12B, 12C of the sensor array 12, only light that is transmitted by the filter may be absorbed and detected by that particular photo-sensor. Sometimes an array of micro-lenses 16 is provided on top of the CFA with a respective micro-lens 16A, 16B, 16C overlying an associated photo-sensor to collect and focus more radiation, thus increasing the sensitivity of the image sensor.
Typical image sensor technologies such as those based on charge coupled devices (CCDs) and complimentary metal oxide semiconductors (CMOS) are less responsive to light in the shorter (i.e. blue and violet) wavelength regions of the visible spectrum compared to the red and green regions of the spectrum. This can in part be due to the high absorption of these wavelengths by the lens material. Additionally, there may a limited penetration depth of this shorter wavelength light at these wavelengths in silicon. Unfortunately, much of the radiation is absorbed at the polysilicon gate region, with very little penetrating into the channel regions of the photo-sensor where the photoelectric signal is generated.
There has been a variety of attempts made in the prior art to alleviate these problems. Some of these attempts have made use of structural modifications that include back-side illuminating thinned devices, pinned photodiodes, and indium tin oxide (ITO) gated CCD sensors. These approaches have achieved relatively good results in terms of short wavelength spectral response, but at the high cost of complicated fabrication. This problem has become a bottleneck to a continuing improvement of dark sensitivity and the color quality of today's digital color cameras which demand inexpensive image sensors.